Triangle Challenger noise when starting up and shutting down

Boiler has been in service for about 3 months and now when it starts up and shuts down there is a snapping or clicking noise which seems to be at the top of the unit. Do you have any ideas what this could be?

It could be the exhaust/intake pipes. If things have slightly shifted, and those pipes run through anything that got tight, as the temp changes, they expand or contract, and if 'caught' on anything, they can easily make clicking sounds. The same thing happens with plastic drain pipes as you say run a shower - you don't normally hear it while showering, but afterwards, as they cool off, it is not uncommon to hear them. The prevention is properly sized holes, and proper support. Harder to fix afterwards.